Halliday & Matthiessen (1999: 398-9):
One manifestation of the second-order nature of the textual metafunction … is grammatical metaphor. Grammatical metaphor is a 'second-order' use of the grammatical resources: one grammatical feature or set of features is used as a metaphor for another feature or set of features; and, since features are realised by structures, one grammatical structure comes to stand for another — with the semantic effects discussed.